1. Field of the Invention
The present invention relates to a process for producing an alloy powder containing rare earth metals. More particularly, it relates to a process for economically producing an alloy powder containing rare earth metals with a low content of oxygen and residual reducing agent.
2. Description of the Prior Art
It is known that alloys (including intermetallic compounds) composed of rare earth metals as a main component are useful for a permanent magnet material, magnetostrictive material, magnetic sensor, magnetic refrigerator, optomagnetic recording material, hydrogen occlusion material, etc.
According to a well-known process, the alloys containing rare earth metals are produced by the steps of preparing respective ingots of rare earth metals and alloying elements (or preparing mother alloys) and melting them using a high-frequency melting furnace.
Where the alloy thus obtained is to be made into a permanent magnet, it is necessary to crush the alloy into fine powder, which subsequently undergoes pressing and sintering. Making a powder by crushing an alloy is disadvantageous because this crushing process is required and rare earth metals are easily oxidized during crushing, adversely affecting the quality of the alloy.
In order to eliminate this disadvantage, there was proposed the so-called reducing diffusion process for the production of alloy powder. This process is now in practical use. According to this process, a rare earth metal-cobalt magnet powder is produced in the following manner. A powder of rare earth metal oxide and a powder of metallic cobalt are mixed with metallic calcium or calcium hydride as a reducing agent. The mixture is heated in an inert gas atmosphere or vacuum, so that the rare earth metal oxide is brought into contact with the melt or vapor of metallic calcium for reduction. Concomitant with reduction, the rare earth metal formed by reduction diffuses into the cobalt particles. Thus there is obtained an alloy powder of uniform composition. The reaction product thus obtained is a mixture of CaO formed as a by-product, unreacted excess metallic calcium, and the desired alloy powder. These components are present in the form of sintered complex mass. When the mass is thrown into water, CaO and metallic calcium changes into Ca(OH).sub.2. Thus alloy powder can be easily separated from Ca(OH).sub.2 which suspend in the water. Residual Ca(OH).sub.2 is removed by washing the alloy metal with acetic acid or hydrochloric acid.
When thrown into water, the mixture mass disintegrates into fine powders on account of the oxidation of metallic calcium by water and the hydration of CaO. This method is economically advantageous because the raw material of rare earth metal is a comparatively cheap oxide, the melting and casting steps are not required, and the crushing step is not required (at least primary crushing is not required). In addition, this method can be applied to not only the production of cobalt alloy powders but also the production of powders of ferro-alloy, nickel alloy, or copper alloy containing a rare earth metal.
The present inventors carried out a series of researches on the application of the above-mentioned reducing diffusion method to the production of a variety of alloy powders containing rare earth metals. As the result, it was found that this method does not provide alloy powders which are satisfactory in particle size and quality for the reasons given below. According to the reducing diffusion method, a mixture composed of particles of rare earth metal oxide, particles of alloying metals, and granules of reducing agent such as metallic calcium is heated in argon or vacuo at 900.degree. to 1300.degree. C. The reaction of a rare earth metal oxide with a reducing agent starts at about 700.degree. C., and the temperature of the reaction product exceeds 1300.degree. C. in a short time because of the exothermic reaction. (The maximum temperature varies depending on the type and amount of rare earth metal oxide used.) This high temperature evaporates the reducing agent having a high vapor pressure, causing the reduction reaction to terminate in a comparatively short time (5 to 12 minutes). On the other hand, continued heating for 1 to 6 hours at 900.degree. to 1300.degree. C. is required for diffusion to provide an alloy of uniform composition. The exothermic reaction and heating at a high temperature coarsen the resulting alloy powder. (The average particle diameter of the resulting alloy powder is greater than twice that of metal powder used as a raw material.) For the production of a powder of uniform composition, it is necessary to perform diffusion sufficiently by raising the heating temperature or extending the heating time, or using a finer metal powder as a raw material.
Diffusion in such a manner, however, results in coarse particles through the binding or metal particles and alloy particles together, and promotes the binding of particles with the oxide of the reducing agent. The resulting mixture mass does not readily disintegrate when thrown into water, and this leads to poor separation of alloy particles from the oxide of the reducing agent. Due to the reasons mentioned above, a finely divided powder of uniform composition is not obtained simply by using a finer metallic powder as a raw material.
The ability of the mixture mass to disintegrate in water is considerably reduced when the content of rare earth metal in the alloy is high or when the metal powder as a raw material is iron or ferroalloy. The reason for this is considered as follows: Concomitant with an increase in the ratio of rare earth metal oxide, it is necessary to increase the amount of the reducing agent. This, in turn, generates more heat and forms more oxides of the reducing agent. In addition, iron powder (or ferroalloy powder) tends to firmly sinter.
The disintegrability in water may be improved to some extent by increasing the amount of reducing agent. Usually the amount of reducing agent added in the reducing diffusion method is 1.1 to 1.5 times the stoichiometric amount necessary for the reduction of rare earth metal oxide. For the improvement of disintegrability, it is necessary to add more than twice the stoichiometric amount. However, increasing the amount of reducing agent is not a drastic solution to disintegrability; rather it leads to an increase in by-product and concomitant loss of alloy powder and also to a cost increase.
In the case where the reaction product does not readily disintegrate, one of the countermeausres is to repeat stirring and decantation and/or to carry out milling or wet milling after the reaction product has been thrown into water. These means are effective in reducing to some extent the amount of residual reducing agent in the resulting powder; however, on the other hand, it produces an adverse effect of increasing the oxygen content due to oxidation reaction and decreasing the yield due to an increased dissolution loss in the subsequent acid treatment. Additional adverse effects are that it is necessary to lower the pH for acid treatment, to increase the number of acid treatment cycles, and to extend the time of acid treatment. The acid treatment under such conditions increases the dissolution of the alloy components. In the case of ferroalloys, the dissolution of Fe leads to a decrease of yield and the dissolved Fe oxides and hydrolyzes to increase the oxygen content in the resulting product.
As mentioned above, the reducing diffusion method for the production of alloys containing a rare earth metal has technical and economical problems. These problems are serious particularly in the cases where the alloy powder contains rare earth metals in a high ratio, the alloy powder contains light rare earth metals (lanthanum, cerium, praseodymium, and neodymium) which are readily oxidized the alloy powder contains Fe as an alloy component, and the alloy powder has a small average particle diameter less than 15 .mu.m. Under these circumstance, there has been a demand for a new process for producing an alloy powder containing a rare earth metal of different kind which meets the requirements for composition, quality and shape.